Recently, most mobile communication terminals (e.g., a mobile phone, a Personal Digital Assistant (PDA), a Portable Multimedia Player (PMP), a gaming tablet, etc.) use navigation devices to provide various services based on location information. Conventionally, coordinates of a mobile communication terminal are indirectly tracked with the aid of a network according to a Global Positioning System (GPS) included in a Base Station (BS). In recent years, however, the mobile communication terminal provides various services by utilizing a navigation device that is included in the mobile communication terminal to directly recognize a location of the mobile communication terminal according to the GPS.
In addition to the GPS operated by the United States of America, a new satellite navigation system called Galileo is scheduled to be implemented sooner or later by the European Union (EU). The whole world will be covered when coordinates are tracked by the new satellite navigation system. Accordingly, in general, most mobile communication terminals are expected to recognize their locations by using signals provided by the satellite navigation system.
Third Generation Partnership Project 2 (3GPP2) synchronization-type Code Division Multiple Access (CDMA) 2000, Institute of Electrical and Electronics Engineers (IEEE) 8021.6e, IEEE 802.16m, WiBro, mobile WiMAX, 3GPP Universal Mobile Telecommunication System (UMTS), 3GPP Long Term Evolution (LTE) systems are examples of mobile communication systems currently or scheduled to be commercialized. In these systems, a Mobile Station (MS) performs a Random Access (RA) process in order to access to a BS. The RA occurs when the MS accesses to a network upon generation of a call connection event while observing downlink paging or broadcasting messages in a state where power is initially on or when the MS registers its location at an arbitrary time.
In general, all MSs randomly perform the access process by sharing a limited uplink channel (or a reverse channel) assigned for the RA. Thus, an initial Transmit (Tx) power level and an access trial interval of each MS is an important factor for successful RA. An access state denotes a state where closed loop power control is not achieved by a BS. In this state, an MS has to decide and determine its initial Tx power without the aid of the BS. If the initial Tx power determined by the MS is significantly large, an RA success probability of the MS can increase. However, the significantly large initial Tx power causes an overhead of the entire wireless network and thus may result in a side effect in which a power level of a reverse interference signal increases. If the initial Tx power is significantly small, an RA signal of the MS may not be delivered to or recognized by the BS, thereby increasing an RA failure probability. Accordingly, the initial Tx power of the MS is generally determined to be slightly less than the level determined by the MS. If there is no response from the BS for the determined initial Tx power, transmission is repeated by slightly increasing the level. The MS determines its uplink Tx power by using a Received Signal Strength Indicator (RSSI) measured in downlink as expressed by Equation 1 below.MathFigure 1P—ini=−P_RSSI−76+NOM—PWR+INI—PWR−16×NOM—PWR—EXT  [Math.1]
In Equation 1, P_ini denotes an initial Tx power level. P_RSSI denotes a received signal strength. NOM_PWR denotes a compensation value for receiving an accurate power level by the BS and has a value in the range of −8 dB to +7 dB. INI_PWR denotes a compensation value, by which a first transmitted value of an RA channel initially has a slightly small value in the BS, and has a value in the range of −16 dB to +15 dB. NOM_PWR_EXT is used for compensation since fading is significant in case of a Personal Communication Service (PCS) system, and has a value of 0 or 1 (in case of a cellular system, 0).
FIG. 1 illustrates a signal flow process for establishing RA and traffic channels between an MS and a BS in a conventional communication system.
Referring to FIG. 1, a BS 100 generates system information in step 110, and transmits the system information to MSs in a cell coverage by using system paging or broadcasting messages in step 112.
When an MS 102 is powered on in step 114, the MS 102 selects a service provider system through which a service is provided to the MS, and performs initialization by obtaining synchronization of the system in step 116. In step 118, the MS 102 receives the system information by monitoring the paging or broadcasting messages, and measures a downlink received signal strength. In step 120, the MS 102 calculates a target Tx power level expressed by Equation 1 above by using the measured downlink received signal strength. Thereafter, if a user attempts a call or if an event such as registration occurs in step 122, the MS 102 attempts access to the BS according to the calculated Tx power in step 124.
In step 126, the BS 100 determines whether the Tx power level of the MS is proper and whether access collision does not occur with respect to another MS in order to examine a possibility of downlink call establishment. If the downlink call cannot be established, the BS 100 transmits a ‘No Channel Assignment Message’ in step 128. Then, the MS 102 recalculates the target Tx power level in step 130, reattempts a call after a specific time elapses in step 132, and reattempts access to the BS 100 in step 134.
In step 136, the BS 100 examines a possibility of downlink call establishment for the MS. If the call can be established, the BS 100 transmits a ‘Channel Assignment Message’ in step 138. Thereafter, the BS 100 establishes uplink and downlink traffic channels with respect to the MS 102 in step 140 and step 142.
The RA may be significantly delayed when an error exists in the Tx power level, wherein the Tx power level is determined directly by the MS by using a downlink RSSI. For example, in a case where the Tx power level is determined by an MS supporting a Frequency Division Duplexing (FDD) scheme according to the downlink RSSI of Equation 1, a channel may experience fast fading, shadowing, surrounding interference signals, etc., and thus a downlink received Signal to Noise Ratio (SNR) may change significantly. In this case, the Tx power level may have many errors.
The 3GPP2 synchronization-type systems may experience quality deterioration in received signals and cell capacity reduction due to mutual interference of BSs in an boundary region between adjacent cells as shown in FIG. 2. In addition, a handover success rate decreases due to mutual interference between the adjacent cells when a handover is performed.
To address the problems above, there is one conventional method in which an MS estimates interference of a control signal or a data traffic signal at a cell edge and then cancels the interference, similarly to a Successive Interference Cancellation (SIC) or Minimum Mean Square Error (MMSE) IC. There is another conventional method in which a control signal provided from a BS for interference cancellation is used to facilitate cancellation of interference of neighboring BSs. In a case where an MS directly cancels interference of the neighboring BSs, there is a limit in performance improvement, and implementation complexity of the MS increases excessively. In case of using the control signal of the BS, the control signal causes an entire cell overhead, and eventually results in the decrease of entire cell capacity. Since cell planning considers only an initial system installation environment in most cells currently in use, quality variation occurs when a new building is built or other systems are installed nearby, which leads to a problem in that information on the quality variation cannot be updated on a real time basis.